Thread: Simple linked list

Hello I do Singly-Linked Lists. But i have probelm. I can´t do some methods (empty methods in my code). How can i do it?
Thank you.
My code:
Main class:

public class Main {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
      Pomoc o1 = new Pomoc();
 
      System.out.println(o1.isEmpty());
      o1.addFirst(10);
      o1.addFirst(20);
      o1.addFirst(25);
      System.out.println(o1.isEmpty());
      System.out.println(o1.getFirst());
 
 
 
    }
 
}

Item class:

public class Item {
    private int data;
    private Item next;
    private Item previous;
 
    public void setData(int data)
    {
        this.data=data;
    }
 
    public int getData()
    {
        return data;
    }
 
    public void setNext(Item next)
    {
        this.next=next;
    }
 
    public Item getNext()
    {
        return next;
    }
 
       public void setPrevious(Item previous)
    {
        this.previous=previous;
    }
 
    public Item getPrevious()
    {
        return previous;
    }
 
 
 
}

Method class

public class Pomoc {
    private Item act;
    private Item first;
 
    public boolean isEmpty()
    {
            return first == null;
    }
 
    public void addFirst(int o)
    {
       Item newItem = new Item();
       newItem.setData(o);
       newItem.setNext(first);
       first = newItem;
    }
 
    public void removeFirst()
    {
        if(first==null)
        {
            System.err.println("Seznam je prazdny");
        }
        else
        {
            this.first=first;
        }
 
        if(act==first)
        {
            act = null;
        }
    }
 
    public int getFirst()
    {
        return first.getData();
    }
 
    public void setActFirst()
    {
        act=first;
    }
 
    public void setActNext()
    {
        Item actNext = new Item();
        actNext.setPrevious(act);
        act = actNext;
 
    }
 
    public void removeAfterAct()
    {
        act.setNext(act);
    }
 
    public void addAfterAct(int i)
    {
        act.setData(i);
        act.setNext(act);
    }
 
    public int getAct()
    {
        return act.getData();
    }
 
    public int getSize()
    {
 
    }
}

The only problem I see is that: public int getSize() is not returning anything, nor is it being used. So you can either edit it out or have it return something. I'm still in the process of learning this stuff, so I might be way off base.

I need know how can i do empty methods...

Is your question that you need to figure out the size of the linked list? Search google and you will get tons of code listing how to do this. An alternative way is to use the java.util.LinkedList. But to do with you class, you must iterate through the list from first to last counting up along the way. Here is a basic way you can do this. It can also be done using recursion, but I won't post that code

public static int getSize(){
	Item f = first;
	int count = 1;//count the first node
	while ( f.next != null ){//if the list continues
		count++;
		f = f.next;
	}
	return count;
}

Oh thnak you very much anf how can i do this method:
public void addAfterAct(int i)
{

}

Thank you so much. I will try do it. But do you have idea how can i do these three methods<
act = actual...

    public void setActNext()
    {
 
    }
 
    public void removeAfterAct()
    {
 
    }
 
    public void addAfterAct(int i)
    {
 
    }

Thank you!

The best way to do the size is probably to maintain a size variable your self and increment it upon a call to add or decrement it upon a call to remove and then all you need to do is return the value of the size variable in your size method.

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